Firefox allows users to restart the browser only when a new add-on or Firefox theme has been installed, however there are several times when the browser performs slow and you may want to restart Firefox rather than having to close and start it again.
A add-on called Restart Firefox adds a option to restart the browser through a menu item. Once you install the add-on, you can restart the browser by visiting the File menu and clicking on the Restart Firefox item.
Definitely useful if you frequently close and start Firefox. The add-on is experimental, so you will need to login to Mozilla add-ons before you can install it in Firefox.